1. Field of the Invention
The present invention relates to high temperature superconductors, and more particularly, to a superconductor incorporating therein a superconductivity epitaxial film having a porous structure and a method for manufacturing the same.
2. Description of Related Art
Electrical resistance, in many applications, is very undesirable. For example, in electrical power transmission, electrical resistance causes power dissipation. The power dissipation grows in proportion to the current in normal wires. Thus, wires carrying large currents dissipate large amounts of energy. It is therefore desirable to fabricate a device that has little or no resistance. Such devices are commonly known as “superconductors.”
Many different materials can become superconductors when their temperature is cooled below the transition temperature Tc. For example, some classical superconductors (along with their respective Tc values in degrees Kelvin (K)) include carbon 15 K, lead 7.2 K, lanthanum 4.9 K, tantalum 4.47 K, and mercury 4.47 K.
In recent years, much research has focused on high temperature superconductors (“HTS”). Many of these materials have superconductive properties at a temperature above that of liquid nitrogen (77 K). Some HTS (along with their respective Tc values in degrees K.) include Hg0.8Tl0.2Ba2Ca2Cu3O8.33 138 K, Bi2Sr2Ca2Cu3O10 118 K, and YBa2Cu3O7-δ93 K. The last superconductor falls under the class of “YBCO” superconductors, based on its components, namely yttrium, barium, copper, and oxygen, and is regarded as the one of the highest performing, high temperature superconductors, especially for electric power applications. See generally Goyal et al., High critical current density superconducting tapes by epitaxial deposition of YBa2Cu3Ox thick films on biaxially textured metal, Appl. Phys. Lett. 69, 1795 (1996); Wu et al., Properties of YBa2Cu3O7-δ thick films on flexible buffered metallic substrates, Appl. Phys. Lett. 67, 2397 (1995); Larbalestier et al., High-Tc Superconducting Materials For Electric Power Applications, Nature 414, 368 (2001).
Among other specifications, the critical current (Jc) is the most critical one for most HTS applications that include high-field magnets, electrical motors, generators and large-capacity power transmission lines. To carry high current, the HTS coatings must have thickness of a few to several micrometers. Unfortunately, the Jc values of conventional YBCO films deposited on both single crystal and bi-axially textured metal substrates experienced a monotonic decrease with increasing film thickness. This Jc-thickness behavior has motivated an extensive effort during the past few years to investigate the related mechanism. See generally Luborsky et al., Reproducible sputtering and properties of Y—Ba—Cu—O films of various thicknesses, J. Appl. Phys. 64, 6388 (1988); Foltyn et al., Pulsed laser deposition of thick YBa2Cu3O7-δ films with Jc≦1 MA/cm2, Appl. Phys. Lett. 63, 1848 (1993); Busch et al., High-quality Y1Ba2Cu3O6.5+xfilms on large area by chemical vapor deposition, J. Appl. Phys. 70, 2449 (1991); Miura et al., Structural and electrical properties of liquid phase epitaxially grown Y1Ba2Cu3Ox films, Physica C. 278, 201 (1997); Foltyn et al., Relationship between film thickness and the critical current of YBa2Cu3O7-δ coated conductors, Appl. Phys. Lett. 75, 3692 (1999); Paranthaman et al., YBa2Cu3O7-y Coated Conductors with High Engineering Current Density, J. Mater. Res. 15, 2647 (2000); Kang et al., Comparative study of thickness dependence of critical current density of YBa2Cu3O7-δ on (100) SrTiO3 and on rolling-assisted biaxially textured substrates, J. Mater. Res. 17, 1750 (2002).
In recent years, researchers have also investigated the effects of small substrate miscuts on the microstructure of thin films. For example, Durrell, Critical Current Anisotropy in High Temperature Superconductors, Dissertation (April 2001), observed small pinholes in a de-oxygenated YBCO thin films grown on a 2°, 4°, 10°, and 20° vicinal substrates. This work all involved thin films less than 200 nm thick, and usually resulted in “pinholes” of varying diameters. The pinholes appear to be shallow holes that appear on the film surface. See also Durrell et al., Critical currents in vicinal YBa2Cu3O7-δ films, Phys. Rev. B 70, 214508 (2004); Durrell et al., Dependence of Critical Current on Field Angle in Off-c-axis Grown Bi2Sr2CaCu2O8 film, Appl. Phys. Lett. 77, 1686 (2000) (120 nm thin film on a 10° vicinal substrate); Durrell et al., Determination of Pinning Forces on Vortex Lines in YBa2Cu3O7-δ, Supercond. Sci. Technol. 12, 1090 (1999) (120 nm thin film on a 6° vicinal substrate); L. Mechin et al., Properties of YBa2Cu3O7δ thin films grown on vicinal SrTiO3 (001) substrates, Physica C 302, 102 (1998) (160 nm thin film on 2°, 4°, and 6° vicinal substrates).
Other groups have observed columnar defects on the order of 2-3 nm in diameter (but no pore formation) in vicinal films deposited after high-temperature pre-treatment of the substrates. For example, in Maurice et al., Effects of Surface Miscuts on the Epitaxy of YBa2Cu3O7-δ and NdBa2Cu3O7-γ on SrTiO3(001), Phs. Rev. B 68 115429 (2003) investigated 200 nm (0.2 μm) thin films deposited on a heat-annealed substrate with a vicinal angle varying between 0.2° and 1.2°. See also Lowndes et al., Strong, Asymmetric Flux Pinning by Miscut-Growth-Initiated Columnar Defects in Epitaxial YBa2Cu3O7-x Films, Phys. Rev. Lett. 74, 2355 (1995) (columnar defects in 0.5 to 1.0 μm films on 2° annealed vicinal substrates); T. Haage et al., Transport properties and flux pinning by self-organization in YBa2Cu3O7-δ films on vicinal SrTiO3(001), Phys. Rev. B 56, 8404 (1997) (24 to 360 nm thin films on annealed 10° vicinal substrates); T. Haage et al., Substrate-mediated anisotropy of transport properties in YBa2Cu3O7-δ thin films, Sol. Stat. Com. 99, 553 (1996) (0.24 μm thin film on 10° annealed vicinal substrate); J. Brotz and H. Fuess, Anisotropic defect structure and transport properties of YBa2Cu3O7-δ films on vicinal SrTiO3(001), J. Appl. Phys. 85, 635 (1999) (60 nm thin films on 10° annealed vicinal substrate); Cantoni et al., Anisotropic nonmonotonic behavior of the superconducting critical current in thin YBa2Cu3O7-δ films on vicinal SrTiO3 surfaces, Phys. Rev. B 71, 054509 (2005) (150 to 250 nm thin films on 0.4°, 4°, and 8° vicinal annealed substrates).
In recent years, a few researchers have also investigated the effect of depositing island-like nanoparticles (as opposed to homogenous epitaxial layers) on the superconducting layer microstructure. See Haugan et al., Island growth of Y2BaCuO5 nanoparticles in (211˜1.5 nm/123˜10 nm)×N composite multilayer structures to enhance flux pinning of YBa2Cu3O7-δ films, J. Mater. Res., 18, 2618 (2003); Haugan et al., Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor, Nature 430, 867 (2004), These experiments were performed on flat substrates, and resulted in improved Jc values at applied magnetic fields.
The effect of Jc on superconducting thick films (greater than 0.5 μm) deposited on vicinal substrates remains unknown, especially as to substrates that have not undergone high-temperature pre-treatment of the substrate. The present invention relates a method of improving Jc in thick films, having a thickness in the range of 0.5 to 3.0 μm, by altering the film microstructure by strain engineering at nanometer scales using vicinal substrates. The resulting superconducting film is porous in nature. Despite the decrease in cross-sectional area, the Jc is unexpectedly increased compared to films deposited on flat substrates. In addition, the present invention demonstrates that by inserting nanoparticles into the superconducting film, the pore density can be increased and pore dimension varied. The insertion of island-like nanoparticles likely releases the strain accumulated with increasing thickness of the vicinal films in a favorable matter, resulting in a higher Jc compared to superconductors prepared without nanoparticles.